linux-kernel - Re: [PATCH 8/8] drm/fourcc: Add DRM_FORMAT

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Date:   Thu, 17 Feb 2022 11:42:29 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        "Linux/m68k" <linux-m68k@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/8] drm/fourcc: Add DRM_FORMAT_D1

Hi Pekka,

On Thu, Feb 17, 2022 at 11:10 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
> On Tue, 15 Feb 2022 17:52:26 +0100
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > Introduce a fourcc code for a single-channel frame buffer format with two
> > darkness levels.  This can be used for two-level dark-on-light displays.
> >
> > As the number of bits per pixel is less than eight, this relies on
> > proper block handling for the calculation of bits per pixel and pitch.
> >
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>

> > --- a/drivers/gpu/drm/drm_fourcc.c
> > +++ b/drivers/gpu/drm/drm_fourcc.c
> > @@ -151,6 +151,8 @@ const struct drm_format_info *__drm_format_info(u32 format)
> >               { .format = DRM_FORMAT_C4,              .depth = 4,  .num_planes = 1,
> >                 .char_per_block = { 1, }, .block_w = { 2, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
> >               { .format = DRM_FORMAT_C8,              .depth = 8,  .num_planes = 1, .cpp = { 1, 0, 0 }, .hsub = 1, .vsub = 1 },
> > +             { .format = DRM_FORMAT_D1,              .depth = 1,  .num_planes = 1,
> > +               .char_per_block = { 1, }, .block_w = { 8, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
> >               { .format = DRM_FORMAT_R1,              .depth = 1,  .num_planes = 1,
> >                 .char_per_block = { 1, }, .block_w = { 8, }, .block_h = { 1, }, .hsub = 1, .vsub = 1 },
> >               { .format = DRM_FORMAT_R2,              .depth = 2,  .num_planes = 1,
> > diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> > index 8605a1acc6813e6c..c15c6efcc65e5827 100644
> > --- a/include/uapi/drm/drm_fourcc.h
> > +++ b/include/uapi/drm/drm_fourcc.h
> > @@ -104,6 +104,9 @@ extern "C" {
> >  #define DRM_FORMAT_C4                fourcc_code('C', '4', ' ', ' ') /* [3:0] C */
> >  #define DRM_FORMAT_C8                fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
> >
> > +/* 1 bpp Darkness */
> > +#define DRM_FORMAT_D1                fourcc_code('D', '1', ' ', ' ') /* [0] D */
> > +
>
> the same comment here as for C1 and R1 formats, need to specify pixel
> ordering inside a byte.

Right, will do.

> I think it would also be good to explain the rationale why C1 and R1
> are not suitable for this case and we need yet another 1-bit format in
> the commit message.
>
> For posterity, of course. I roughly remember the discussions.

C1 is color-indexed, which can be any two colors.
R1 is light-on-dark.
D1 is dark-on-light.

> I also wonder if anyone would actually use D1. Should it be added
> anyway? There is no rule that a pixel format must be used inside the
> kernel AFAIK, but is there even a prospective userspace wanting this?
>
> Exposing R1 and inverting bits while copying to hardware might be
> enough?

That's an option.  The repaper driver does that:

    drm_fb_xrgb8888_to_gray8(buf, 0, cma_obj->vaddr, fb, &clip);
    repaper_gray8_to_mono_reversed(buf, fb->width, fb->height);

Can drm_framebuffer objects be backed by graphics memory, i.e.
can they be displayed without copying?

> >  /* 1 bpp Red */
> >  #define DRM_FORMAT_R1                fourcc_code('R', '1', ' ', ' ') /* [0] R */

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds